Testing for Life on Mars

The Viking missions returned confusing results, but a new hypothesis about microbes that live in presence of hydrogen peroxide provides a possible explanation.
Credit: NASA / JPL / Malin Space Sciences System

More than 30 years ago, NASA’s Viking life-detection experiments on Mars returned inconclusive results, and controversy surrounds the Viking data to this day. The Phoenix mission now on Mars may collect data that will help answer questions raised by Viking, but to do so researchers will need to be able to spot the difference between chemical and biological signatures on Mars.

In the mid 1970s, NASA’s Viking landers analyzed material from the martian surface in the hopes of finding signatures of life. The Viking life-detection experiments indicated the presence of oxidized organic material near the surface. At first, researchers on the instrument team believed this was evidence of biological activity on Mars.

However, other scientists immediately criticized the results. Eventually, the scientific community decided that it was more likely that organic material had been oxidized by the chemical hydrogen peroxide (H2O2), which was detected at the surface by the Viking Gas Exchange experiment.

A layer of water ice coats the rocks and soil at the Viking Lander 2 landing site, Utopia Planitia.
Credit: NASA

Not everyone is satisfied with this explanation. There are still questions about where hydrogen peroxide on Mars comes from, how long it can naturally remain stable at the surface, and how much is actually present.

An answer may be on the horizon. NASA’s Phoenix lander touched down on Mars on May 25. Onboard is an instrument called the Thermal and Evolved Gas Analyzer (TEGA), which will be used to analyze soil and ice samples on Mars. The instrument could help explain the puzzling Viking results.

In order to prepare for the upcoming Phoenix experiments, a team of research performed a series of tests. They simulated the experiments to be conducted by Phoenix to determine what the results would look like if microbes were present in martian soil. Researchers inoculated simulated martian soil with known Earth-organisms that can survive in the presence of hydrogen peroxide. (On Earth, some microbes produce hydrogen peroxide and others are able to utilize it.)

Mixtures of water and hydrogen peroxide freeze at much lower temperatures than water alone. The researchers theorize that microbes might be able to use hydrogen peroxide to survive at lower temperatures. The idea is based on Earth microbes that use salts in a similar way, providing them with an ‘anti-freeze’ that keeps them alive in cold environments. If microbes on Mars adapted to use hydrogen peroxide in a similar way, it may mean that the Viking results need to be reinterpreted.

A model of the Viking landers sent to Mars by NASA in the 1970s.
Credit: NASA

By testing the simulant martian soil with the TEGA instrument, the research team was able to identify signatures that indicated microbes. Specific chemicals that can be detected by TEGA can now serve as biomarkers for possible life on Mars.

The simulated experiments provide an important test for chemical versus biological responses in the Phoenix mission. In other words, the Phoenix mission could provide definitive answers to the questions posed by Viking, and may even tell us if life exists on Mars.

The research was published in the April 2008 edition of the journal Astrobiology under lead author Dirk Shulze-Makuch.